The invention relates to stability monitoring for a lift vehicle and, more particularly, to longitudinal stability monitoring for lift vehicles such as telescopic material handlers, front end loaders, and container handlers (stakers) that is determined using a rear axle load.
Lift vehicles serve to raise loads or personnel to elevated heights. For example, a telescopic material handler (telehandler) is a wheeled construction machine that carries loads to elevated heights or different locations. Such a machine tends to tip forward when overloaded or when its telescopic boom is lowered or extended at a fast rate. Stability requirements for telehandlers are controlled by the market in which they are sold. All markets share common static stability requirements that are performed on a tilt bed. Dynamic stability requirements caused by boom movement, on the other hand, vary depending on the market. In 2008, the controlling regulatory agencies in Europe introduced a new standard that requires the machine to have the intelligence and capability to stop itself in case of impending instability considering forces due to boom dynamics.
Operators of these machines prefer fast boom functions (lift up, lift down, telescope out and telescope in) so they can do more work in less time. Manufacturers tend to provide these speeds by not limiting the hydraulic system capability. Also, these boom function speeds are usually tested and documented without a load on the machine forks.
Machines generally do not have the capability to distinguish between a loaded and unloaded status, and therefore, boom function speeds stay the same whether the machine is loaded or unloaded. Experienced operators handle this situation well by adjusting the boom speed (using boom functions controlled by a joystick or the like) based on boom length and on what capacity is on the forks. Although mistakes are rare, they still happen when an operator engages the control joystick in a way that causes the boom to lift-down at a rate that makes it possible to tip the machine if a load monitoring would stop the function. It would be desirable for a longitudinal monitoring system to deal with such cases and reduce the probability of tipping.
Lowering boom function speeds was the easy solution to such a dynamic problem. Simulation results showed that the telescope-out function speed is not critical for forward tipping, and the focus should be on the lift-down function. The question then was how slow the boom lift-down speed should be to prevent tipping while operating at any point in the machine load chart. For each machine, a simulation was performed for normal lift-down with constant speed and for lift-down with sudden stops at different locations in the work envelope. Simulation results showed that to prevent tipping at any point in the load chart, current machine speeds need to be slowed down by a factor of two to three times depending on the class of the machine (max height and max capacity). Since the machine has no capability to distinguish between loaded and unloaded conditions, this simple solution was deemed unacceptable because these slow speeds would be too limiting for the machine performance particularly when it is unloaded.